The present invention relates to an arrangement for the handling of white water from a paper machine, in which a channel is employed having an inlet end connected to an outlet for white water from a paper machine and having an outlet end that defines a main outlet for the white water to discharge into a container such as a white water silo. The invention relates more particularly to such an arrangement wherein the channel has a bottom that slopes downward in the flow direction with a certain angle of inclination relative to horizontal that is at least as great as a certain critical angle of inclination in order to avoid sedimentation on the channel bottom.
In the wet end of a paper machine large amounts of water are drained from the pulp suspension or stock during formation of a fiber web. In very large machines the flow rate of this drained water can be 1 m3/s. This liquid, commonly called white water, must be handled in a rational and cost-efficient manner. Typically, almost all of the white water is collected and is then recirculated for use in other parts of the papermaking process, such as for dilution, washing of the pulp, etc. It is necessary that the air content of the white water be relatively low in order to be able to reuse the white water.
Suitably, the removal of air is made in the simplest possible way, i.e., by giving the white water a sufficiently long period of time for the air bubbles to rise to the surface and thus be removed from the water before the water is reused. For this purpose, a so-called white water channel is provided in a paper machine. The channel has a certain length and width as well as a certain flow rate all of which are selected to give the desired deaeration. Normally, the flow rate in the white water channel should not exceed 0.5 m/s to obtain good deaeration. Further, it is important to avoid turbulence, as there is otherwise a risk that the air bubbles will rotate down into the water again. In order not to disturb the flow mode, sedimentation on the bottom of the white water channel should be avoided. To avoid such sedimentation, the white water channel is made to slope downward at a sufficient angle of inclination in the flow direction that fibers and other particles are carried to the outlet rather than settling on the bottom of the channel. Experience shows that the critical angle of inclination should be at least 2xc2x0, preferably at least 3xc2x0, in order to avoid sedimentation. In many installations, however, there is no possibility to provide the white water channel with a sufficiently large inclination, for instance when the height of the white water outlet from the paper machine is relatively low in combination with the white water silo being located at a large distance from the white water outlet of the paper machine. To address this problem, special cavities or recesses have been created in the machine floor of the plant on which the paper machine stands, so that sufficient inclination is obtained at the bottom of the channel that goes to the silo, which is located on a lower floor below the machine floor. In some cases, it has been necessary to remove essential portions of the machine floor in order to obtain sufficient inclination. It will be recognized that such a solution is not desirable from an economic point of view in that it is expensive to cut away concrete material and to construct a suitable channel arrangement therein. Furthermore, such a solution is not desirable from the standpoint of safety because the removal of part of the floor can affect the stability/safety of the building.
An arrangement in connection with a white water channel is disclosed in DE-29916787 U1, the purpose of which is to prevent admixture of air caused by whirls in the white water channel. The white water channel is divided into two entirely separate sections, which are located at different levels, one above the other. The white water can flow down from the upper section to the lower one through a plurality of outlet pipes located at different levels. Through this arrangement, the creation of whirls and hence admixture of air in the white water channel are said to be prevented. There are, however, several drawbacks with such an arrangement. Firstly, because of the division into different levels, this design requires special conditions as to the plant building in order to make it possible to employ such an arrangement at all. It is not always desirable that such an adaptation of the plant building should be made in order to eliminate air problems in the white water channel. Further, the design is likely to entail increased costs for outlets, etc., which is not desirable. Finally, it is uncertain whether such an arrangement would be effective for the removal of the small bubbles that are formed in connection with the paper web formation and that follow the white water into the white water channel. As already mentioned, there is an advantage if the many small bubbles that are formed in connection with the web formation are removed from the water before it is recirculated.
The present invention addresses the above needs and achieves other advantages, by providing an arrangement for handling white water from a paper machine, in which the critical angle of inclination of the white water channel can be achieved even when the running distance of the channel is relatively long considering the height difference between the paper machine""s white water outlet and the main outlet of the channel. In accordance with one embodiment of the present invention, the arrangement comprises a white water channel having an inlet end for receiving white water from a paper machine and an opposite outlet end defining a main outlet through which the white water is discharged into a container, the channel having a channel bottom, the channel bottom sloping downward in a flow direction from the inlet end to the outlet end. In accordance with the invention, an additional outlet is provided between the main outlet and the inlet end, the bottom having a first bottom portion that leads away from the inlet end up to the additional outlet and a second bottom portion that leads away from the additional outlet toward the main outlet. The first and second bottom portions are so arranged in relation to each other that each of the bottom portions slopes downward in the flow direction at an angle of inclination that exceeds a critical angle of inclination that avoids sedimentation on the bottom.
The invention thus can eliminate the need to make changes in the existing floor surface on which the paper machine stands, and can reduce the cost of the construction.
Preferably, the additional outlet is formed through the bottom of the channel. It is further preferred that the first bottom portion slope downward from an upstream end to a downstream end thereof and the additional outlet is located substantially at the downstream end of the first bottom portion.
In accordance with another preferred embodiment of the invention, the second bottom portion slopes downward from an upstream end to a downstream end thereof, and the upstream end of the second bottom portion is at a higher vertical level than the downstream end of the first bottom portion.
A particularly preferred embodiment has the upstream ends of the first and second bottom portions located at substantially the same vertical level.
In another preferred embodiment, the bottom of the channel includes an intermediate bottom portion between the first and second bottom portions, the intermediate bottom portion sloping upward from a downstream side of the additional outlet to the upstream end of the second bottom portion. The intermediate bottom portion has an angle of inclination larger, and preferably substantially larger, than that of either of the first and second bottom portions, for example between 5xc2x0 and 60xc2x0.
Each of the first and second bottom portions has an angle of inclination exceeding about 2xc2x0, more preferably exceeding about 2.5xc2x0, most preferably exceeding about 3xc2x0. The angles of inclination of the first and second bottom portions can be approximately equal.
In one embodiment of the invention, the bottom of the channel has a V-shaped configuration in transverse cross-section and the additional outlet is located at a lowest point of the transverse cross-section.
It is also within the scope of the invention for the channel to include a third bottom portion following the second bottom portion and also sloping downward in the flow direction with an angle of inclination exceeding the critical angle of inclination. In this case, a second additional outlet is formed in the channel between the inlet end and the main outlet.